1. Industrial Useful Field
This invention relates to an improvement of an automatic transmission and especially to an automatic transmission which is designed to improve a power transmission efficiency at each speed ratio zone.
2. Prior Art and Its Problem
First in case of a passenger car, corresponding to a manual four-speed speed changer a three-speed automatic transmission is used, which combines a three-element, one-stage type torque converter and a three-speed speed changer having a planetary gear train.
This three-speed speed changer has two pairs of planetary gear trains, two clutches and two brakes, and is so adapted as to change speed in forward three speeds and backward one speed by automatically controlling the clutches and brakes.
Incidentally, in a conventional automatic transmission, transmission characteristics of torque converter are transmitted to a rear-stage speed changer at a constant condition over the entire speed ratio, and are similarly changed speed by the speed changer.
Further, as has been known, the torque converter has such a characteristic that a turbine torque larger than a pump torque (large torque ratio) is outputted as a turbine rotation speed becomes smaller than a pump rotation speed (low speed ratio).
On the other hand, since a vehicle requires a largest torque when being accelerated, the above-mentioned torque converter characteristic will function effectively in a low speed ratio zone but a driving power will become insufficient in a frequently-used intermediate speed ratio zone and further in a high speed ratio zone.
Therefore, in a conventional automatic transmission, its torque converter is so designed that characteristics of torque converter may develope a good torque and excellent efficiency in the intermediate speed ratio zone.
In such a torque converter, however, it is inevitable to minimize a torque generated in the low speed ratio zone due to the above-mentioned reason, and a stall torque ratio is generally about 2 so that a sufficient torque output can not be obtained at the time of accelerating the vehicle.
For this reason, the vehicle speed is changed into three steps using a speed changer. However, since the speed changer is equipped with two clutches and two brakes for carrying out an automatic control, its mechanism becomes complicated to require a troublesome control.
Next in case of an automatic transmission for use in truck, a tractive force larger than that of passenger car is required so that a four-speed automatic transmission is used, which combines a three-element, one-stage torque converter and a four-speed speed changer having a large reduction ratio corresponding to a six-speed of manual speed changer.
In this speed changer, since it is necessary to automatically control three pairs of planetary gear train by the use of two clutches and three brakes, the automatic transmission becomes more complicated than the case of passenger car.
Further in case of an automatic transmission for use in a construction machine, a torque converter having a large stall torque is combined with a three-speed or four-speed power shift speed changer in order to further increase a tractive force. However, this type of tranmission includes a problem of worse fuel consumption due to an insufficiency in torque in the intermediate speed ratio zone and a deterioration of efficiency.